Electrical power for an integrated circuit (IC) is typically supplied by one or more direct current (DC) power sources, such as a buck-mode, pulse width modulation (PWM) DC-DC converter of the type diagrammatically shown in FIG. 1. In the illustrated buck-mode converter, a DC-DC controller 10 switchably controls the turn-on and turn-off of a pair of power switching devices, respectively depicted as an upper power MOSFET device 20 and a lower power MOSFET device 30. These power MOSFET switching devices have their drain-source paths coupled in series between first and second bias supply voltages (VCC and ground (GND)). A common or phase voltage node 25 between the two power MOSFETs 20/30 is coupled through an inductor 40 to a capacitor 50, which is coupled to a reference voltage terminal (GND). The connection 45 between inductor 40 and capacitor 50 serves as an output node from which a desired (regulated) DC output voltage Vout is derived.
The buck converter's DC-DC controller 10 includes a pair of gate driver circuits 11 and 12, which controllably turn respective switching devices 20 and 30 on and off, in accordance with a pulse width modulation (PWM) switching waveform produced by a comparator 13. The upper MOSFET device 20 is turned on and off by an upper gate switching signal UG applied by the gate driver 11 to the gate of the MOSFET device 20, and the MOSFET device 30 is turned on and off by a lower gate switching signal LG applied by the gate driver 12 to the gate of the MOSFET device 30.
To produce the PWM waveform, comparator 13 compares the signal level of a periodic reference waveform, such a sawtooth signal supplied by a sawtooth generator 14, with a reference voltage output by an error amplifier 15. The frequency of the PWM waveform corresponds to that of the periodic waveform supplied by generator 14, while the duty cycle of the PWM signal is controlled by the output of the error amplifier 15. For this purpose, the error amplifier 15 compares a fraction of the output voltage Vout at the output node 45, as derived by voltage divider 16, and coupled through a soft start circuit 17, with prescribed reference voltage 18. As further shown in FIG. 1, the DC-DC controller 10 may include an overcurrent detector 19 coupled via resistor 23 to the VCC bias voltage terminal, and to node 25. A shut down circuit 22 is controlled by the output of the overcurrent detector, so as to controllably interrupt operation of the power supply in the event of an overcurrent condition.
In a number of situations, it may be necessary to provide one or more operating voltages that are different from the available supply voltage on a single card. In at least one application, such as a dual-data-rate (DDR) memory system employing DDR random access memories (DRAMs), two supply voltages are required. Typically, a second supply voltage will be a prescribed fraction (e.g., one-half) of a first supply voltage, and generally may not exceed the first supply voltage.
For improved integration density, where the DC power supply is regulated by an integrated circuit, it is desirable to combine control functions for the multiple power supplies in a single IC. Although implementing a multi voltage supply may be a technical challenge, the benefits of efficiency encourage its pursuit. One straightforward way to configure a dual voltage converter is to simply fabricate two discrete circuits of the type, such as that shown in FIG. 1, on a common motherboard. A similar approach is described in the U.S. Pat. No. 6,067,241 to Q. Lu, entitled “Dual-Output DC-DC Power Supply.” Lu proposes cascading two types of DC converters—a forward DC-DC converter and a buck converter—in order to realize a ‘half-brick’ sized power supply.
Now although such a ‘doubled’ DC converter architecture may provide two different voltages, each supply is effectively a discrete, stand-alone circuit, having its own dedicated controller. This fact, coupled with the large size and complexity of the soft start and overcurrent detection circuitry for each converter, make the resulting multi voltage supply configuration relatively complex, expensive, as well as requiring a significant amount of chip area. Moreover, implementing a pair of discrete converters of the type described in the Lu patent is problematic at best, due to its use of a forward DC-DC converter circuit, which contains a transformer. As such, this type of DC power supply architecture is not practical for powering highly integrated electronic components, such as DDR DRAMs, and the like.